The human immune system comprises numerous different types of cells having overlapping functions which together act to protect the human body against sickness and disease. The cells of the immune system have complex multiple functions and interconnecting relationships. A major component of the immune system that plays an essential role in protecting the host against infection by these organisms is the humoral antibody.
Antibodies, also known as immunoglobulins, are protein molecules which have exquisite specificity for the foreign particle which stimulates their production. Immunoglobulins (Ig) are a class of structurally related proteins consisting of two pairs of polypeptide chains, one pair of light (L) low molecular weight! chains (.kappa. or .lambda.), and one pair of heavy (H) chains (.gamma., .alpha., .mu., .delta., and .epsilon.), all four linked together by disulfide bonds. Both H and L chains have regions that contribute to the binding of antigen and that are highly variable from one Ig molecule to another. In addition, H and L chains contain regions that are nonvariable or constant.
The L chains consist of two domains. The carboxy-terminal domain is essentially identical among L chains of a given type and is referred to as the "constant" (C) region. The amino-terminal domain varies from L chain to L chain and contributes to the binding site of antibody. Because of its variability, it is referred to as the "variable" (V) region.
The H chains of Ig molecules are of several classes, .alpha., .mu., .delta., .alpha., and .gamma. (of which there are several subclasses). An assembled Ig molecule, consisting of one or more units of two identical H and L chains, derives its name from the H chain that it possesses. Thus, there are five Ig isotypes: IgA, IgM, IgD, IgE, and IgG (with four subclasses based on differences in the H chains, i.e., IgG1, IgG2, IgG3, and IgG4).
Not all antibody isotypes are equal in the performance of the many biological roles of antibodies. For example, IgA, primarily present in secretions such as tears, urine, saliva, colostrum, sweat, and mucus (i.e., secretory IgA) is the major immunoglobulin synthesized in the body. It is believed to be the primary immunological defense against local infections in such areas as the respiratory or gastrointestinal tract. Secretory IgA is also an efficient antiviral antibody, preventing the viruses from entering host cells, as well as an efficient agglutinating antibody.
The actions of the very versatile and long-lived IgG ranges from neutralization of toxin to activation of complement and opsonization. For example, IgG reacts with epitopes on microorganisms via its Fab portions leading to the final engulfing and destruction of the microorganism. It also plays an important role in antibody-dependent, cell-mediated cytotoxicity (ADCC) by activating natural killer cells which then destroy the target by release of various substances. It is also an effective virus neutralizing antibody.
In contrast to IgG, IgM antibodies are not very versatile; they are poor toxin-neutralizing antibodies, and they are not very efficient in the neutralization of viruses. IgM is found predominantly in the intravascular spaces and is the isotype synthesized by children and adults in appreciable amounts after immunization or exposure to T-independent antigens, and is the first isotype that is synthesized after immunization with T-dependent antigens. IgM molecules are the most efficient agglutinating and complement-activating antibodies.
IgD molecules, present on the surface of B lymphocytes, appear to be involved in the differentiation of these cells and generally have not been demonstrated to serve a protective function.
IgE, also called reaginic antibody, is of paramount importance in hypersensitivity or allergy reactions. These reactions may be mild, such as in the case of a mosquito bite, or severe, as in the case of bronchial asthma. The reactions may even result in systemic anaphylaxis, which can cause death within minutes.
Despite this variability, all immunoglobulins derive from antibody-secreting cells. The precursors of the antibody-secreting cell are B lymphocytes, also known as "B cells." B cells are a type of lymphocyte that derives from hematopoietic stem cells by a complex set of differentiation events that are only partially understood.
B cells bear as a cell-surface receptor an immunoglobulin (Ig) molecule specialized for expression on the cell surface. Newly differentiated B cells initially express surface Ig solely of the IgM class. Associated with maturation of a B cell is the appearance of other immunoglobulin isotypes on the surface of the B cell.
To release antibody, the B cells must first be activated. There are many ways to activate B cells, including cross-linkage of membrane (m) Ig molecules by the antigen mIg (cross-linkage-dependent B cell activation), direct encounter with T cells (helper T cells or helper T cell-associated molecules, such as, for example, CD40 ligand), or encounter with mitogens. In such encounters, the antigen presents epitopes recognized by the B cell's cell-surface Ig.
Because each B cell bears multiple membrane Ig molecules with identical variable regions, high level cross-linkage of the cell-surface receptors yields optimal activation. This cross-linkage requires that the antigen present more than one copy of the epitope that the cell-surface Ig recognizes. Although many simple protein antigens do not have this potential, polysaccharides and other antigens with repeating epitopes, such as surfaces of microbes and DNA, do. Among these more complex antigens are the capsular polysaccharides of many medically important microorganisms, such as pneumococci, streptococci, and meningococci.
There are much data to show that cross-linkage of membrane Ig can also lead to elimination or inactivation of B cells. In general, it is believed that certain types of receptor cross-linkage events, if they occur in the absence of specific stimulatory signals, lead to inactivation rather than activation. The highly repetitive epitopes expressed on polysaccharides may lead to activation in the absence of costimulation, possibly because of the magnitude of the receptor-mediated stimulation.
Once the B cells which initially express IgM and/or IgD on their surface, are activated the antibodies that are initially produced are primarily of the IgM isotype. Most interestingly, in response to a specific challenge, T cell help allows B cells to "class switch" their particular V region to an appropriate C region (i.e., to a region corresponding to .gamma., .alpha., and .epsilon.). Thus, a cell that expresses receptors of the IgM and IgD classes may differentiate into a cell that expresses IgG, IgA, or IgE receptors and then into a cell that secretes those antibodies. This process allows the production of different antibodies with their distinct biological functions against the same antigen that first induced the antibody response.
The induction of the switching process depends upon the action of a specialized set of B cell stimulants. Studies have identified two different kinds of stimuli that are important for the induction of isotype switching. The first kind of stimuli are predominantly responsible for the specificity of the isotype-switch event. These include, but are not limited to, the cytokine IL-4, interferon-.gamma. (IFN-.gamma.), and transforming growth factor-.beta. (TGF-.beta.). They are all characterized by the ability to induce the transcription of an immature (germline) form of RNA for the heavy chain isotype to which switching will be induced. Stimuli that induce switching to one isotype often stimulate switching to an additional isotype and inhibit switching to other isotypes.
The second kind of stimuli, or combinations of stimuli, activates B cells and often results, by themselves, in B cell proliferation and Ig secretion without isotype switching. These stimuli may permit switching to occur to any of a number of different isotypes, depending on which of the specific stimuli are present, although some of the stimuli appear to block switching to some isotypes. These permissive stimuli include, but are not limited to, bacterial lipopolysaccharide (LPS), the cytokine IL-5, crosslinking of cell surface Ig, signals given by molecules on the cell membranes of activated T cells (for mouse B cells), infection with Epstein-Barr virus (EBV), crosslinking of CD40 ligand (a cell surface protein found generally on a specialized group of T cells), crosslinking of the tumor necrosis factor-.alpha. (TNF-.alpha.) receptor, and signals given by molecules on the cell membranes of activated T cells. Of these, the most widely studied are LPS (acting in the mouse but not in the human) and direct interaction of CD40 ligand.
Despite this information, the parameters necessary for induction of high-rate IgA class switching were unknown prior to this invention. Because IgA antibodies are an important line of defense against infection and disease, there is a need in the art for this knowledge. Such knowledge facilitates additional and superior methods of treating immune disorders.